For children (5-12yrs) from low-income households, summer vacation is a ?window of vulnerability? and represents an extended period of time (typically 9-10 weeks) in which declines in academic performance occur. For 30 years, empirical evidence has indicated that children from low-income households experience greater declines in reading and math skills during the summer vacation than their middle-to-upper income peers. Additionally, a growing body of evidence demonstrates that the amount of weight gained during summer vacation is 3-5 times greater than the amount of weight gained during the school year. Moreover, this excessive weight gain during summer is more pronounced for low-income, minority children, the same children experiencing the greatest summer learning loss. What children eat and drink, the types of physical activity opportunities, and the amount of screen-time and sleep (two important correlates of weight gain) they engage in during summer vacation is unknown. However, a substantial body of literature indicates that children engage in a greater number of obesogenic behaviors during less-structured times (e.g., weekend days) compared to more-structured times (e.g., week/school days). We refer to this phenomenon as the Structured Days Hypothesis. These behaviors include 1) increased time spent sedentary, 2) reduced engagement in physical activity, 3) displaced sleep patterns, and 4) unhealthy dietary patterns. This study hypothesizes that summer vacation is simply one long weekend in which the effect of children?s obesogenic behaviors on weight are compounded over a three-month time period. However, should children be involved in a structured program over the summer they would not engage in obesogenic behaviors and would not experience unhealthy weight gains and fitness loss. Recently, one school district in the Florence, SC area adopted a year-round calendar for an elementary school. This provides a unique opportunity to conduct a natural experiment to examine the effects of shorter periodic breaks vs. one prolonged break from structured days (i.e., school days) on obesogenic behaviors, weight gain, and fitness. We will use a two-arm accelerated longitudinal cohort design to complete the following specific aims: Aim 1: Compare changes in BMI z-scores and fitness during the traditional 3-month summer and 9-month school year between children attending a school that follows a year-round school calendar and children attending match-paired schools that follow a traditional school calendar. Aim 2: Compare changes in sleep, physical activity, sedentary behaviors, and diet during the traditional 3-month summer and 9-month school year between children attending a school that follows a year-round school calendar and children attending match-paired schools that follow a traditional school calendar. This study is significant because the reasons for children?s weight gain and fitness loss during the summer are unknown. This study is innovative because no studies have examined the effects of prolonged (9 weeks all at once) vs. periodic breaks (3-week breaks distributed throughout the year) from a structured environment on children?s weight and fitness.